Variables

1. Variables Art &Technology, 3rd SemesterAalborg University Programming https://art.moodle.aau.dk/course/view.php?id=33 David Meredith dave@create.aau.dk

2. Reading • Chapter 4 of Shiffman, Learning Processing.

3. Overview • Variables • Types • Variable declaration and assignment • Incrementing variables • System variables • Generating random numbers • Type casting

4. Variables x y z intx = 3; inty = 5; intz = x + y; x = 6; z = x + y; • When a program runs on a computer, it processes information that is stored in its memory • Think of a computer’s memory as consisting of a huge array of boxes in which we can store information • We’re able to assign names to these boxes, so that if we want to get the information stored in a box, we just have to refer to the box by its name • A labelled memory “box” like this is called a variable, because the information in it (its value)can vary • We have to create (or declare) a variable before we use it by giving it a name and stating what type of data we can store in it (e.g., integers, decimal numbers, strings of characters, etc.) 3 6 5 8 11

5. Types • The type of a variable is the kind of data that it can hold, e.g., • whole numbers (int, byte, short, long) • e.g., … -2, -1, 0, 1, 2, … • floating-point numbers (double, float) • e.g., 3.14159, -2.5, 0.4, 2.0, … • characters (char) • e.g., ‘a’, ‘b’, ‘c’, ‘*’, ‘€’, ‘2’, etc. • boolean (boolean) • e.g., true, false

6. Declaring type and initializing • When we create a variable, we must define what its name is and what its type is: intx; //Declares a variable called x of type int double d; //Declares a variable called d of type double char letter; //Declares a variable called letter of type char • Can (and should) also initialize a variable when we create it, by storing some value in it //Declare a variable called x of type int, initialized to 0 intx= 0; //Declare a variable called d of type double, initialized to 0.0 double d= 0.0; //Declare a variable called letter of type char, initialized //to ‘a’ char letter = ‘a’;

7. Using variables • If you want a variable to be available everywhere in the program, declare it before the definition of setup(), at the beginning of your file

8. Assigning a value to a variable: The ‘=‘ symbol • We assign a value to a variable by using the ‘=‘ operator, thus: x= 5; // Sets the value of x to 5 x= 7; // Sets the value of x to 7 • Remember that ‘=‘ means “becomes equal to” so x= 5; means “xbecomes equal to 5” • If we want to increase (increment) the value of a variable by 1, then we write: x = x + 1; // “xbecomes equal to x plus 1”

9. Incrementing a variable to animate an image • Initialize circleX to 0 so circle starts at left • draw() is executed on each frame • circleX is increased by 1 on each frame • So circle moves to the right

10. The circle moves and grows in size • diameter of circle stored in diameter variable • diameter increased by 1 on each frame

11. Lots of variables

12. System variables • width – the width of the sketch output window • height – the height of the sketch output window • frameCount– the number of frames processed by the sketch so far • frameRate– the number of frames per second • screen.width– the width of the screen • screen.height– the height of the screen • key – most recent key pressed • keyCode– a numeric code for the last key pressed • mousePressed– true if the mouse button is pressed, false otherwise • mouseX– current x-coordinate of mouse position • mouseY– current y-coordinate of mouse position • pmouseX– x-coordinate of previous mouse position • pmouseY– y-coordinate of previous mouse position • mouseButton– the mouse button that was last pressed (LEFT, RIGHT, CENTER)

13. Using system variables

14. Drawing an ellipse with variablesWhich program produced which image?

15. random() – a function that returns a value • When you call the functions line(), ellipse() & rect(), you just get something printed to the output window • The function random() gives you a random number: float w = random(1,100); This line of code generates a random floating point number between 1 and 100 and stores this value in the float type variable called w We can then do stuff like rect(100,100,w,50); This line of code draws a rectangle with the top left corner at (100,100), a width equal to the random number stored in w and a height equal to 50 pixels

16. random(low, high) and random(high) • random() returns a floating point value, so random() is defined somewhere as follows: float random(float low, float high) { … } • You can also call random() with just one argument – it then returns a random floating point number between 0 and the number you give it as its argument: float w = random(10); This line of code generates a random floating point number between 0 and 10 and stores this value in the float type variable called w

17. Type casting • random() returns a float – but what if you really want an int? intw = int(random(1,100)); generates a random float between 1 and 100, then converts this to an int and stores it in the int type variable, w

18. Random ellipses Note use of system variables, width and height, to control position and size of ellipses